1. Field of the Invention
The present invention relates to an optical information reading device for reading optical information such as a bar code with a light beam such as a laser beam that is caused to scan the optical information by a torsion resonator that resonates at its own resonating frequency in a self-resonating mode.
2. Description of the Prior Art
Optical information reading devices, such as bar code readers, are known. The optical system of one such known bar code reader is shown in FIG. 1 of the accompanying drawings. AS shown in FIG. 1, a laser beam emitted by a laser beam source 81 is directed through a semireflecting mirror 87 to a polygonal mirror 83. The polygonal mirror 83, which rotates about its own axis at a constant speed, reflects and deflects the laser beam to scan a bar code label 85 printed with a bar code of any of various standards such as the UPC (Universal Product Code) bar code, Code 39, ITF (Interleaved 2 of 5), etc. The bar code label 85 reflects off the applied laser beam back to the polygonal mirror 83, which also reflects the reflected laser beam to the semireflecting mirror 87. The semireflecting mirror 87 directs the reflected laser beam through a condensing lens 89 toward a photodetector 91. The bar code reader shown in FIG. 1 is disclosed in Japanese laid-open patent publication No. 63-269263, for example.
The laser beam that is applied from the laser beam source 81 to the polygonal mirror 83 and the reflected laser beam that travels from the polygonal mirror 83 to the photodetector 91 are separated from each other by the semireflecting mirror 87. Therefore, the optical system of the bar code scanner is relatively large in size, making it difficult to reduce the overall size of the bar code scanner.
Japanese laid-open patent publication No. 2-178888 discloses another bar code reader which has a scanning mirror for applying a laser beam from a laser beam source to a bar code label, and a converging mirror for directing a laser beam reflected by the bar code label toward a photodetector. The scanning mirror and the converging mirror are reciprocally angularly moved about a common axis by a step motor or the like.
Since the laser beam source and the photodetector can be disposed in substantially the same position, the optical system of the bar code reader can be simpler than that of the bar code reader with the semireflecting mirror. As a result, the bar code reader can be reduced in side and weight.
However, since the single motor is employed to angularly move the scanning mirror and the converging mirror, a relatively large load is imposed on the motor, and hence the motor cannot be reduced in size. The large load on the motor results in a relatively large electric power requirement for the motor.
Quartz Light Scanner For Laser Printer, written by Sugiyama et al. Preprint for 28th SICE Scientific Lecture Meeting, pages 581.about.582, 1989, discloses a crystal resonant scanner using a torsion resonator. The disclosed resonant scanner is shown in FIG. 2 of the accompanying drawings. As shown in FIG. 2, the resonant scanner, generally denoted at 60, has a torsion resonator 58 composed of a mirror 50, a coil unit 52 with a coil (not shown) formed thereon, a torsion bar 54, and a fixed support 56, the torsion resonator 58 being made of crystal by photolithography and anisotropic etching. The resonant scanner 60 also has a pair of spaced magnets 59a, 59b for applying a magnetic field to the coil unit 52 disposed between the magnets 59a, 59b for thereby torsionally vibrating the torsion resonator 58 back and forth in the directions indicated by the arrows A.
A laser beam emitted from a laser beam source 62 is applied through a collimator lens 64 to the mirror 50 which, when vibrated back and forth, deflects the applied laser beam to scan a desired surface such as a bar code label 66. When the coil of the coil unit 52 is supplied with an alternating electric current of a certain frequency, the laser beam reflected by the mirror 50 scans the bar code label 66 periodically at the frequency of the electric current supplied to the coil unit 52. Inasmuch as the resonant scanner 60 employs no electric motor, the resonant, scanner 60 may be smaller in size and weight and consumes a reduced amount of electric energy. If the frequency of the electric current supplied to the coil unit 52 is selected to be equal or close to the resonant frequency of the torsion resonator 58, then the torsion resonator 58 can resonate at its resonant frequency, resulting in a much lower power requirement than the conventional motor-driven light beam scanners. The resonant scanner 60 is suitable for use as a hand-held, portable optical information reader.
As shown in FIG. 2, a laser beam reflected by the bar code label 66 is applied through a condensing lens 68 to a photodetector 70, which produces an electric signal representative of the intensity of the laser beam falling on the photodetector 70. The electric signal generated by the photodetector 70 is then supplied to an electric signal processing system. More specifically, the electric signal from the photodetector 70 is amplified by an amplifier 72 into an amplified signal, which is then converted into a binary signal by a binary signal converter 74. The binary signal is decoded by a decoder 78 into a decoded signal that is sent to an output terminal 79. The coil of the coil unit 52 is energized by a mirror driver 80 for turning the mirror 50 back and forth to deflect the laser beam. The laser beam source 62 is energized by a laser driver 82. The decoder 78, the mirror driver 80, and the laser driver 82 are controlled by a controller 84. The decoder 78 and the controller 84 are in the form of an electronic control unit comprising a microcomputer having a CPU, a ROM, and a RAM.
The resonant frequency of the torsion resonator 58 is relatively high. Therefore, when the torsion resonator 58 is caused to resonate at its resonant frequency, i.e., in a self-resonating mode, the resonant scanner 60 scans the bar code label 66 at a very high rate, at least ten times the scanning rate of the conventional light scanner with a polygonal mirror. According to Quartz Light Scanner For Laser Printer, referred to above, the torsion resonator 58 has a resonant frequency of 840 Hz, and the scanning rate of the resonant scanner 60 in the self-resonating mode is about 1600 scans per second. The high scanning rate requires the signal processing system to have a high signal processing capability.
In reality, the controller 84 controls the mirror driver 80 to supply the coil unit 52 with an electric current of a frequency that is selected to oscillate the torsion resonator 58 at a frequency lower than the resonant frequency thereof. In such a modes however, the resonant scanner 60 consumes more electric power than in the self-resonating mode, cancelling out the advantages offered by the torsion resonator 58.
The resonant frequency of the torsion resonator 58 may be lowered when the mirror 50 is increased in size and weight. An increase in the size and weight of the mirror 50 causes the resonant scanner 60 to be larger in size, and necessitates an increased cost for the manufacture of the torsion resonator 58. The large-size mirror 50 is also susceptible to external shocks. For these reasons, the size of the mirror 50 cannot be increased for the purpose of lowering the resonant frequency of the torsion resonator 58.
More specifically, the impedance Zs and resonant frequency fo of the resonant scanner 670 are expressed as follows: ##EQU1## where RDC is the resistance of the coil, r is the damping coefficient, C is the torque coefficient, j is the moment of inertia, and k is the torsion spring constant.
The torsion spring constant k is determined by the torsion bar 54 of the torsion resonator 58, and the moment j of inertia by the size and weight of the mirror 50. Therefore, if the torsion bar 54 remains unchanged, the resonant frequency fo can be lowered when the size and weight of the mirror 50 are increased. As described above, if the mirror 50 is increased in size and weight, then the resonant scanner 60 is increased in size, and the torsion resonator 58 is manufactured at an increased cost. The large-size mirror 50 is susceptible to external shocks.